This invention relates to a method for manufacturing a color filter with an arrangement of filter elements of a plurality of colors for displaying a color image or for separating an image into a plurality of colors and providing image signals of these colors.
Recently, a color display device using a color filter in which filter elements of three primary colors, blue, green and red are arranged, has been put into practical use. In this device, the light rays coming through filter elements of these colors are synthesized for forming an image. A color image sensor has also been put into practical use. This device uses a color filter comprising filter elements of red, blue and yellow or filter elements of cyan, magenta and yellow and/or filter elements of black and white, and generates image signals of these colors.
In this type of the color filter, color filter elements are regularly arranged in a stripe fashion or a mosaic fashion, corresponding to the electrodes for display and photo detecting elements. Each filter element is extremely small. In the case of the stripe-type color filter, the width of each filter element is only about 0.1 mm or so. For this reason, photolithography is used for manufacturing the color filter, since this technique provides a minute pattern with high accuracy, and provides high productivity.
An example of a manufacturing process of the color filter by photolithography will be described with reference to FIGS. 1A and 1B. As shown in FIG. 1A, photosensitive film 3 is formed on glass substrate 1 on which electrodes 2 have already been made. Photo mask 4 is positioned above photosensitive film 3. Photo mask 4 comprises glass substrate 5 and opaque metal (made of, for example, chromium) layer 6 is formed on the portion of the surface of substrate 5, on which a filter is not to be formed. In the second step, photosensitive film 3 is irradiated by ultraviolet rays (UV) through photo mask 4, to expose the portion of the filter below the light transmitting portion 4a of photo mask 4. As a result of this radiation of ultraviolet rays, only portion 3a, which defines a shape of the filter element, is photochemically changed by an optical reaction. Then, in the developing process, the optically-unreacted portion, or unexposed portion 3b, is removed. In this way, the exposed portion 3a of photosensitive film 3 is formed on the electrodes 2 of glass substrate 1. This exposed portion is used as dyeable material layer F. By dyeing the dyeable material layer for each color, a color filter element of each desired color can be obtained.
In the conventional method of manufacturing a color filter as described above, if photo mask 4 is defective, or if dust is deposited on photo mask 4, the following problem arises. This problem will be described using a case where dust is deposited on the portion 4a of a desired filter pattern of photo mask 4. For this description, reference is made to FIGS. 1A and B. Portion 3c of photosensitive film 3 below dust 7, which is deposited on photo mask 4, is not radiated by ultraviolet rays in the ultraviolet rays-exposure step, since the rays are blocked by dust 7. This means that portion 3c is not optically reacted and is, therefore, removed in the developing process. As a result, this portion fails to become dyeable material layer F, forming a pinhole 8 in dyeable material layer F. Since pinhole 8 is located in the filter just above electrode 2, and at this pin hole portion, no light is absorbed by the filter element, pinhole 8 will appear as quite a bright white spot. In the color display device using such a color filter, because of the white spot, the quality of an image is extremely deteriorated. In the color image sensor using this type of color filter, color separation ability is degraded, and the image signal obtained has poor image reproducibility.
For the above reason, to manufacture such minute patterned filters, an environment is selected in which the size of dust particles in the air is limited to a certain value or less, and the number of these dust particles is also limited. To remarkably decrease the number of dust particles in the air, however, larger manufacturing facilities are required and the manufacturing cost is increased. Besides, it is almost impossible to eliminate dust altogether.
As for the mask manufacturing, the manufacture of defect-free masks leads to an increase in manufacturing costs and is actually almost impossible.
As described above, the conventional manufacturing method of a color filter has a serious disadvantage in that, due to defects of the photo mask and/or dust on the photo mask, numerous pinholes occur in the color filter.